This invention relates to a method and system for non-contact measurement of a gap between a sensor and a conductive or non-conductive surface using a capacitive measurement device with a plurality of conductive plates that also permits measurement of gas, material depth and dielectric changes in solids and fluids.
Non-contact gap measurement sensors having two parallel superimposed conductive plates, which are electrically insulated from one another, are disclosed in, for example, U.S. Pat. Nos. 4,675,670; 5,990,807; 6,075,464 and 6,552,667. A high frequency signal is placed on the first plate (sense plate) of the sensor. By measuring the capacitive interaction between the sense plate and a proximate surface, the sensor generates a signal that is indicative of the gap or dielectric between the sensor and the surface.
An active guard plate is located behind the sense plate to prevent the sense signal from interacting with surfaces that lay behind the sense plate. Interaction between the sense signal and any surface except the desired proximate surface of interaction produces an error in the expected output. For the same reason it is also necessary to prevent the sense signal that is carried within the cable from interacting with any surfaces that are not at the same potential.
Non-contact capacitive sensors may be used in environments of high voltages and currents. For example, these sensors may be attached to a stator of a power generator to measure a gap between the stator and a rotor. Within the generator, the electromagnet field intensity may reach in excess of 15000 gauss. Under these conditions, strong eddy currents can form on the metal surfaces of a sensor probe. These eddy currents, if not curbed, may generate sufficient heat to damage the sensor probe and the generator.
To minimize eddy currents, it is well-known to laminate conductive materials of the generator such as the copper windings and the magnetic poles. Similarly, to minimize eddy currents on a conductive sheet as is used within a capacitive sensor, it is well-known to etch closely-spaced and parallel grooves on the metal surfaces. These grooves are often referred to as “combing” in that the grooves appear as the teeth of a hair comb. The grooves block eddy currents on a metal surface by forming dielectric gaps on the surface. The grooves may be filled with resin and fibers from the material, e.g., epoxy, used to bond the metal plates together in a sensor.
A difficulty is that the electric field signal on the sense plate passes through the combing grooves of the active guard to surfaces that lay behind the active guard. This leakage current through the guard plate may introduce a measurement error. In addition the sense signal carried on the center conductor of the coaxial cable connected to the sensor may pass through the braided coaxial active-guard layer because of the voids between the metallic strands of the cable.
Another difficulty created by the combing of the active guard on the sensor and the voids between the metallic conductors of the active-guard layer of the cable is that variations in the combing width and strands may cause inconsistent signal errors from sensor to sensor and cable to cable. The combing and strand variations arise from manufacturing variations.
There is a need for a capacitive measurement method and a non-contact capacitive measurement sensor that is less sensitive to variations in capacitance due to manufacturing variations in the sensor probe plates and cabling between the probe and a proximity circuit. Excessive sensitivity to these variations may increase the difficulty in manufacturing the sensor and increase the sensor sensitivity to temperature and other environmental factors.